The general activity level of a patient has been recognized in the art as useful for providing a programming signal for a variable rate cardiac pacer. For example, U.S. Pat. No. 4,140,132 suggests the use of a timing device having a self-generating voltage source to modify the constant rate of a cardiac pacemaker. That patent provides a device which embodies a cantilever suspended element which constitutes a high impedance voltage generator, such as a piezo-electric element which vibrates when subjected to motion, to provide alternating voltage from the resulting strain upon it. The cardiac pacer timing pulse rate is thereby varied as a rate of physical activity as measured by the piezo-electric element.
U.S. Pat. No. 4,428,378 suggests the use of an activity sensor mounted within a pacer. The activity sensor detects the general activity level of the patient and alters the escape interval of the pacer between a preset minimum and maximum in response to the detected activity level of the patient. U.S. Pat. No. 4,545,380 also suggests the use of a piezo-electric device for setting or adjusting parameters or functions of an implanted device such as a cardiac pacer in response to the user's impact near the implanted piezo-electric device. In yet another U.S. Pat. No. 4,771,780, it is suggested to use a motion sensor mounted within a rate responsive pacemaker. The motion sensor includes an enclosed housing having a conductive element therein that partially fills the space of a cavity within the enclosed housing. The conductive element is free to roll, flow or otherwise move around the inside of the housing in response to external forces. The types of sensors used in prior art devices are high in cost and present very complex design problems compared to the present invention.
The present invention has features and advantages not found in the prior art. This invention provides a variable rate cardiac pacer apparatus responsive to the physical activity of the patient including an electrode interface sensor apparatus which actually senses the physical movements of the pacemaker housing relative to its position in the body. The apparatus provides an impedance signal which can readily be processed into control signals for modifying the pacer pulse rates.